Teaching Energy Concepts by Working on Themes of Cultural and Environmental Value
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- Besson, U. & De Ambrosis, A. Sci & Educ (2014) 23: 1309. doi:10.1007/s11191-013-9592-7
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Energy is a central topic in physics and a key concept for understanding the physical, biological and technological worlds. It is a complex topic with multiple connections with different areas of science and with social, environmental and philosophical issues. In this paper we discuss some aspects of the teaching and learning of the energy concept, and report results of research on this issue. To immerse science teaching into the context of scientific culture and of the students’ cultural world, we propose to select specific driving issues that promote motivation for the construction of science concepts and models. We describe the design and evaluation of a teaching learning path developed around the issue of greenhouse effect and global warming. The experimentation with high school students has shown that the approach based on driving issues promotes students’ engagement toward a deeper understanding of the topic and favours further insight. The evolution of students’ answers indicates a progressively more correct and appropriate use of the concepts of heat, radiation, temperature, internal energy, a distinction between thermal equilibrium and stationary non equilibrium conditions, and a better understanding of greenhouse effect. Based on the results of the experimentation and in collaboration with the teachers involved, new materials for the students have been prepared and a new cycle of implementation, evaluation and refinement has been activated with a larger group of teachers and students. This type of systematic and long term collaboration with teachers can help to fill the gap between the science education research and the actual school practice.
A considerable amount of educational research has been devoted to the teaching and learning of energy concepts and phenomena. Many studies have pointed out students’ common conceptions that can create learning difficulties and different approaches for teaching energy have been designed and experimented. In this paper we briefly review the literature on students’ conceptions on energy, summarize some approaches to teaching energy proposed by science education research, and then we propose an approach aimed at integrating the Science Technology Society Environment (STSE) approach with the conceptual and procedural dimensions of science learning. In this perspective we have developed a teaching learning path, devoted to high school students, around the problem of understanding the physical basis of greenhouse effect and global warming. We wanted to strictly connect the environmental aspects and the scientific content, and we paid particular attention to the conceptual progression and connections with basic energy concepts: differentiating the concepts of work, heat, internal energy, temperature; considering the role of radiation in thermal phenomena; understanding energy conservation and energy balances in stationary situations of thermal non-equilibrium.
The sequence has been tested in six high school classes, four with 17–18 year old students and two with 15–16 year old students for a total of 121 students. We investigated two research questions: (1) how can the study of a complex issue such as greenhouse effect and global warming improve understanding of energy concepts and (2) what type of materials, experiments, models and schematic representations can favor students’ understanding of this topic. In the present paper we describe the main features of the learning sequence, and the methods, data sources and results of its implementation by a group of teachers.
2 Students’ Conceptions and Conceptual Difficulties on Energy
Many researches have been devoted, since the eighties, to the teaching and learning of energy concepts and phenomena and to students’ common conceptions.1 Research shows that anthropocentric and vitalistic conceptions are prevalent in youngest pupils: energy is connected to life, to movement and to the capability of doing actions, while it is not related to non-living and motionless objects, except for objects explicitly devoted to storing and supplying energy, like batteries and fuels. The idea that energy can transform in different forms, largely conveyed by textbooks, is often meant as a kind of metamorphosis, like in fairy tales, rather than a conservation principle of a measurable physical quantity.
Some researchers (Watts 1983; Viennot 2001; Goldring and Osborne 1994) point out that students merge in an undifferentiated notion the concepts of energy, force and momentum, using them almost as synonymous. Actually, an undifferentiated idea of force-energy has been used by scientists for a long time, while a clear distinction of the physical meaning of the two terms stemmed only in the second half of the nineteenth century. Helmholtz, in 1847, entitled OntheConservation of Force (Krakt in German) his famous work where he established a general energy conservation principle and the expression vis viva (living force), introduced the first time by Leibniz, was used in high school textbooks as synonym of kinetic energy until few decades ago.
For high school and college students, research shows the tendency to consider energy as something producing actions and effects and thus consuming itself, rather than to use energy conservation and degradation to explain phenomena.2 Students show difficulties in distinguishing between extensive and intensive quantities, in particular between heat and temperature: they often use a mixed temperature-heat notion, or consider the temperature of an object as a measure of the level of heat. Students speak about “heat contained in a body” as a substance that can pass from a body to another, in a way that remembers the ancient conception of caloric, and confuse heat with internal energy. Sometimes both heat and cold are considered as substances that can be transferred from a body to another one.
Work is generally not connected to temperature changes, while the prevalent idea is that only heat exchanges can increase or decrease the temperature of an object: this idea survives also in university students and can be found among science teachers too.
Some of the students’ difficulties with the idea of heat derive from the difference between its meaning in common language and in the language of science. This concept is often used as synonymous of internal energy or at least of thermal energy, i.e., of the part of internal energy connected to the changes of temperature. Difficulties in differentiating the meaning of heat, work and internal energy hinder the understanding of the first law of thermodynamics and in general of the energy conservation principle. The language generally used in textbooks does not clarify the ambiguity of common language because expressions like heat supplied convey the idea that a body possesses heat in order to give it. These remainders of the old conception of heat as fluid, not explicitly addressed, are critical from the educational point of view because they can prevent a correct understanding of these concepts.
On the other hand, any distinction between heat and work disappears at the microscopic level: interactions are of the same type, the difference is in their coherence or incoherence and appears only at macroscopic or mesoscopic level where a large number of molecules are involved (Besson 2003). For that reason, expressions like mechanical work and thermal work could be properly used.
Learning progression of energy concepts across middle school grades has been studied by Hee-Sun Lee and Ou Lidya Liu (Lee and Liu 2010). Based on the results obtained with a sample of 2,688 middle school students in different schools and in different states, the authors conclude that “students’ overall knowledge integration levels with energy concepts are mediocre, that advanced energy concepts such as conservation are more difficult than identifying energy sources and that the origin of this difficulty is in part related to the increased demand for integrating many scientifically relevant ideas”.
If you want to think up a good noun for ‘energy transferred by virtue of a temperature difference’ that would be fine with me. Call it Harry, call it Quincy, anything except heat”. Try it out on me next time you have occasion to submit a thermodynamic AJP paper.
Baierlein (1999, p. 18) simply affirmed:
The gain or loss of internal energy, equal to the difference between the energy of the thermal radiation which is absorbed and that which is radiated, is called heat.
In the history of physics the terms radiant heat have been used for a long time even if Maxwell wrote:
Energy that is being transferred by conduction or radiation may be called ‘heat’. That is a technically correct use of the word and, indeed, a correct use as a noun.
Often in textbooks radiation is presented as a way of heat transmission, and this implicitly suggests the idea that radiation and heat have the same characteristics, thus contributing to students’ difficulties. The historical development of the idea of radiant heat and the study of its characteristics compared to the properties of light shows how the process of differentiation was a long one and required both experimental and theoretical efforts (Besson 2012). This suggests that it is worthwhile to explicitly address the problem with the students to promote a good understanding of the energy conservation principle and of the first law of thermodynamics. More specifically, we think that from an educational point of view it is worthwhile to distinguish between thermal conduction and radiation as two ways of transferring energy. According to the terminology suggested above, we could say radiative transfer of energy to be added to mechanical and thermal transfer of energy.
The phrases radiation of heat and radiant heat are not quite scientifically correct, and must be used with caution. Heat is certainly communicated from one body to another by a process which we call radiation. We have no right, however, to speak of this process of radiation as heat… when we speak of radiant heat we do not mean to imply the existence of a new kind of heat but to consider radiation in its thermal aspect. (Maxwell 1871, pp. 15–16)
3 What is Energy?
The question can have a double meaning: one can ask about the essence of energy, or about an operational and mathematic definition of energy as physical quantity.
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas. (R. Feynman Lectures on Physics, Vol. I, p. 4-2).
As far as the first aspect is concerned, one could say that the question has nothing to do with physics, as other similar questions about “what electricity is, or what time is?”. Nevertheless, the ontological character of energy is questionable. In the past both the conception of energy as a kind of substance and as an abstract quantity, invented by scientists to describe phenomena, were supported by different scientists. Attributing a substance to physical quantities is a spontaneous tendency of common sense that can be found sometimes also in scientists’ thinking (this is suggested for example by expressions like “an electric charge q is moving in an electric field” or “let’s take a mass m” often used in exercises and questionnaires). Energy, like a substance, is an extensive quantity and satisfies a conservation principle. Nevertheless, it depends on the frame of reference in the kinetic energy contribution, a property that does not fit well with the idea of substance. Moreover it is additive only if the interaction fields are considered as constitutive parts of the system. Unlike other extensive and conservative variables, such as momentum, energy evokes images, suggestions and meaning far beyond the framework of physics contents.3
Recently the New Age literature has adopted the term energy in its representations and descriptions introducing the ideas of fluxes of positive or negative energy, of energetic auras, energetic vibrations and so on. A physicist could distance her/himself from all these polysemic implications as not pertinent to her/his work and not necessary for describing physical phenomena or to solve problems. On the contrary, a teacher cannot ignore this kind of images and suggestions that some students spontaneously associate to the concept or to the word energy.
But this contradicts the second law of thermodynamics and the theory of heat engines (Lehrman 1973): two bodies at different temperatures can produce work if used appropriately in a thermal engine, but they lose this capacity reaching thermal equilibrium without losing energy. The ability to do work can be expressed by the free energy or by the quantity exergy,4 (Ogborn 1986; Viglietta 1990). Hicks (1983) holds that the definition of energy as capacity to do work “should not be used even as an initial definition, even with remarks to its inadequacy”.
The energy of a body may be defined as the capacity which it has of doing work, and is measured by the quantity of work which it can do. (Maxwell 1871, p. 90)
Many researchers have underlined that the capacity of producing changes is related to the existence of differences, of disequilibrium and that the variable measuring the distance from equilibrium, and then the possibility of changes, is entropy (in mechanics the tendency of potential energy toward a minimum must be considered too).
Energy is not the ‘go’ of things… the possession of energy is not what drives, explains or account for change… entropy or free energy is what decides if the change can happen.
One can also say that the transfer of energy from a body to another or the transformation from one form to another is a measurement of the change occurred [a similar definition is given by Hecht (2007)]. Following a famous analogy proposed by Feynman in his Physics Lectures, many scholars prefer a general definition of energy as a scalar quantity that is conserved in any physical process and transformation. This is a definition in term of properties, rather than a constructive and operational one, and implies that the conservation principle become the postulate of existence of energy itself: a type of ontological statement very common in mathematics, but unusual in experimental sciences.
One can conclude that in teaching any simple and short definition of energy should be avoided; in fact, like other physical quantities, such as temperature or mass, energy is a quantity that cannot be defined by a unique sentence, it needs a progressive construction of meaning and it can be really understood only after using it in different contexts and problems.
4 Approaches to Teaching Energy Proposed in Science Education Research
Different teaching approaches to energy and related phenomena can be found in science education research and in textbooks. We briefly present in the following some examples of the themes and solutions proposed.
Some researchers propose a historical approach, developing a didactical reconstruction of the historical development of the energy concept and conservation principle by showing the process leading to the definition of the concepts as they are now, through debates, changes, errors and progressive conceptual alignment. The historical reconstruction can be mainly focused either on the development of the ideas and of the scientific and philosophical debate (all changes but something remains constant, nothing is created nor destroyed, the unity of nature…) or on the technological, economical and social issues (typically, the birth of the industry, the measure of human work, the steam engines and the industrial revolution).
For example, Coelho (2009) designed a path following some stages of the development of the energy concept from Mayer, Joule and Thomson to Maxwell and Planck. The author argues that interpreting, in the light of Mayer’s and Joule’s studies, the principle of energy conservation as an equivalence principle could prevent the reference to energy as something that cannot be created neither destroyed, but only transformed and that, in this sense, must be a real thing, a substance. The author remarks that many physics textbooks in the last decades of the nineteenth century and at the beginning of the 20th used a formulation of this type, and defined the first law of the thermodynamic as the “equivalence principle”.
Baracca and Besson (1990) proposed a historical introduction of the energy concept more centered on technological and social issues. The underlying idea is that the meaning of the word ‘work’ in common language and in real life has to be taken into account rather than disregarded. Therefore it is useful and necessary to develop a discussion and a reflection on the historical origin and the motivations of the use of such word to indicate a particular physical quantity. Starting from the economic need, in the newborn industry, of measuring human work and of comparing it with the work made by animals and by machines, the path presents the evolution of the definitions of work and mechanical energy together with the problem of the optimization of the movement transmission (Smeaton, Lazare Carnot). Subsequently, the problems related to the introduction of steam engines in the industrial revolution are discussed and the theories of heat, the relationships between heat and work and the idea of thermodynamic efficiency are presented.
A proposal of informal education based on a thorough historical analysis was realized by Bevilacqua and Falomo (2010) in a historical interactive Laboratory on energy. The laboratory presented the historical development of energy concept through the reconstruction, the analysis and the interpretation of historical experiments, available to visitors in an interactive exhibition open to students and teachers and to general public.
Other authors propose a STSE (Science Technology Society Environment) approach focusing on problems related to the use of technology, the availability of energy resources, the preservation of the environment, the social and economic issues, with a multidisciplinary perspective, often including activities involving the outside school context. For example, Domenech et al. (2007) stressed the importance of taking into account the relationships between science, technology, society and environment, and consider that science education research on energy has given little attention to these aspects and to the problem of students’ motivation. According to them, in students’ difficulties not only conceptual, but also procedural and values aspects are involved and it is necessary to face the problem in a global way, avoiding approaches dealing with single aspects of the energy issue. They consider it necessary to show how the energy concept is used in different scientific areas, and to encourage the students consider new problematic situations.
Concerning the conceptual sequence in introducing energy concepts, different solutions can be found in handbooks and in research works.
Most high school physics textbooks, propose a gradual and progressive conceptual sequence, following the usual separation of physics subjects in the curricula: at first, within the topic of mechanics, are given the definitions of work, kinetic and potential energy and subsequently the energy conservation is progressively extended to other physics phenomena, so that the general concept of energy is constructed step by step. A variation in the same frame is to start the construction of the energy concept from thermal phenomena, and then to show the equivalence between heat and other quantities connected to energy.
On the other hand, a different path can be followed which presents from the outset a general concept of energy as a scalar quantity obeying a law of conservation. In this case, different situations are studied and compared concerning a variety of phenomena (mechanical, electromagnetic, thermal, chemical, nuclear…), with examples of energy balances in heat engines, electrical motors, power stations or other practical devices. Two examples of this type of conceptual sequence, which can be defined a holistic approach to energy, are given by Solbes et al. (2009) and Papadouris and Constantinou (2011).
Solbes et al. (2009) present the concepts of energy conservation and degradation as cross-sectional ideas for all physics topics, and design and experiment a teaching sequence with this objective for upper secondary school students.
Papadouris and Constantinou (2011) propose an approach to energy whose main objective is developing the understanding of the nature of science (NOS) as an important aspect of scientific learning. The energy concept is introduced as a “theoretical framework” invented by scientists in order to describe the changes that happen in physical systems. Within this framework the students are guided to gradually recognize the fundamental features of energy (transfer, conservation, degradation) and to use them in the study of various situations. The didactic approach is based on a close correlation between the evolution of the knowledge on the nature of science and the conceptual understanding, with the hypothesis that these two aspects can support each other.
4.1 Our Approach
Observation of school practice, and the cooperation with in-service teachers, together with specific research results, led us to the conviction that it is necessary to overcome a too de-contextualized and technical approach to physics teaching. In particular, as far as energy issues are concerned, it is necessary to immerse physics contents into the context of scientific culture by discussing different interpretations and conceptions which caused historical debates, sometimes not completely resolved, even today. This means to match ideas and references of a STSE (Science Technology Society Environment) approach with a special attention to conceptual, procedural and ethical dimensions of the scientific learning.
For this aim we propose to select specific driving issues which can promote motivation towards a progressive construction of physics concepts and models. This approach also allows teachers and students to better clarify the scope and value of physics concepts and to connect them to other disciplines and to the students’ cultural context. There is a wide literature documenting the advantages offered by using driving questions and project-based learning in promoting students’ motivation and understanding of fundamental aspect of science (see for example Blumenfeld et al. 1991; Krajcik et al. 1998; and Edelson et al. 1999). Studies have been developed to design and test methodological and technological supports favoring students’ engagement in investigations rich enough to promote motivation and interest (Reiser et al. 2001; Reiser 2004; Sandoval and Reiser 2004).
In this line we have developed a teaching–learning path, devoted to high school students, addressing the specific problem of the greenhouse effect and global warming and aimed to develop in this context a better understanding of energy and energy conservation concepts (Besson et al. 2010a).
Anomalous greenhouse effect and global warming are central issues in public debate since the eighties (see IPCC 2007). Nevertheless, in addressing the global warming as a socio-cultural issue the details of the physical aspects are very often neglected and the physical basis of the phenomenon is treated in a hurried way to stress mainly the environmental and economical aspects. So, the physical content and the general social issue remain quite separate from each other and a true understanding of the phenomenon is not favoured. Our aim is to tightly connect and match together the two aforementioned aspects, thus providing the needed conceptual basis. The two aspects should interweave and support each other so that the consciousness of the social relevance of the topic can provide motivation to deepen the conceptual aspects and the conceptual understanding can supply the necessary tools to follow the public debate critically, assume aware points of view, develop knowledge autonomously and assume decisions.
The design of the teaching path is based on a ‘three-dimensional approach’ (Besson et al. 2010b) which involves a synergic integration of three aspects: a critical analysis of the scientific content in view of its importance for teaching, an overview of current treatments (textbooks, common teaching) and an analysis of didactic research on the topic (common conceptions of the topic and teaching learning sequences).
5 Students’ Conceptions and Common Learning Difficulties on Thermal Radiation and Greenhouse Effect
Common conceptions on heat and temperature, phase changes, and thermal conduction have been studied together with the development of these ideas after teaching.5 A number of researches explored students’ conception on light, vision and colours, and learning sequences on these topics have been produced.6
On the other hand, few researches have been carried out on students’ ideas about thermal effects of radiation and the existing ones focus only on particular aspects. Redfors (2001) studied university students’ reasoning on the interaction between radiation and metals involving atomic models and quantum theory. Some researchers investigated students’ understanding of phenomena involving X-rays and radioactivity by focusing in particular on interaction with living organisms and on risks for man and environment (Lijnse et al. 1990; Millar 1994; Rego and Peralta 2006).
Other studies dealt with the understanding of the greenhouse effect in connection with the problem of global warming of our planet7 and some researchers also propose ideas and approaches for dealing with common students’ difficulties (Rye et al. 1997; Meadows and Wiesenmayer 1999; Koulaidis and Christidou 1999).
the ozone layer depletion and radioactivity are often indicated as causes of global warming and the skin cancer is considered as an effect of global warming;
the idea of ‘trapping’ of Sun rays by atmosphere is used as explanation of the greenhouse effect;
the higher temperature inside the greenhouses is explained as the consequence of a non-steady state in which more energy enters than exits.
The relationship between students’ science content knowledge on greenhouse effect and global warming and their awareness of social activism has been studied at different age levels and activities have been designed to favor students’ understanding of the connection between personal energy use and global warming.8 These studies confirm the necessity of carefully designed science curriculum to develop a coherent understanding of global warming and help students make informed decisions about their personal lifestyle choices.
a tendency to give absolute meaning to optical properties (transparency, absorptivity and emissivity) as intrinsic characteristics of bodies;
a lacking or incorrect consideration of infrared emission of bodies as a mechanism of losing energy;
a confusion between transitory phases, in which temperature changes, and steady state situations;
a difficulty in taking into account the systemic interdependence of all factors and phenomena implied in energy budgets.
Concerning the greenhouse effect, only 6 % of student teachers and 9 % of high school students gave an enough correct explanation in terms of solar radiation absorbed and of thermal radiation emitted by the ground, 35 % proposed the idea of ‘trapping’ of sun rays inside the greenhouse and 31–39 % reasoned only in terms of thermal isolation due to atmosphere. The problem of Earth global warming was often confused with that one of ozone layer depletion or “ozone hole” and its origin was generically attributed to pollution, injurious gases, deforestation, human heating.
6 Greenhouse Effect, Climate Change and Energy Concepts
Concepts and ideas related to energy are critical elements for understanding mechanisms of greenhouse effect and global warming. Conversely, the problem of global climate change can provide motivation stimuli, rich context and productive field of interest for pointing out and clarifying concepts related to energy. Central points are the ideas of transmission and transformation of energy, of energy conservation and balance, and the basic phenomena of interaction between radiation and ordinary matter (reflection, absorption, transmission and emission). At this purpose it is necessary to point out that when radiation meets ordinary matter the energy is split up in energy of reflected, transmitted and absorbed radiation, and a balance must exist between these energies and the energy of incoming radiation. All the discourse and all explanations on this topic at school level can be handled in a phenomenological way by means of these concepts only.
A widespread shortcoming, which can be found not only among students but also in textbooks and in didactic research articles, is the lack of consideration of radiation emission and absorption by atmosphere and greenhouse gases and confusion between absorption and reflection. The process of absorption of radiation (and the consequent increase of internal thermal energy) is separated and independent of the emission of thermal infrared radiation, which occurs always and for all bodies, independently of the presence of incoming energy. They are two distinct phenomena and this is shown by the fact that the emitted radiation is uncorrelated and different from the absorbed radiation. On the contrary, reflected radiation is immediately produced when the object is touched by the incoming radiation; it is of the same type (equal frequency) and correlated to the incoming radiation. This distinction must be clearly discussed with students and attention must be paid to the language used. It is misleading to speak of transformation of solar radiation into Earth infrared radiation and of Earth infrared radiation transformed into atmosphere infrared radiation or of re-emission of infrared radiation by the atmosphere. It is essential to introduce the idea that the three processes of reflection, absorption and transmission involve all objects in different proportion, and depend on the materials and on the spectrum of the considered radiation (Besson 2009). It is necessary to stress that emission of radiation occurs always for all objects in an amount and with a spectrum which depend on the temperature and on the surface features.
For all these reasons the analogies between atmosphere (and greenhouse gases) and a blanket and/or a shield are misleading (Svihla and Linn 2012). We think that it is more useful to consider simple cases, for example the behaviour of one or more glass sheets, in order to show by observations and experiments the presence of reflected, transmitted, absorbed and emitted radiation, to well distinguish them, and to represent the related energy balances by using opportune graphs. Subsequently, the glass example can be used as model for more problematic cases, such as greenhouse gases, atmosphere, Earth surface, clouds, ice, glaciers, oceans…
The temperature of an object (system) is connected in a complex way to its internal energy. Only a part of this last is strictly related (is proportional) to the temperature and this part of internal energy is often called thermal energy.
It is possible to increase the temperature of a system by increasing its internal energy. This can be brought about both by doing work on it (heating without heat) or by giving heat (heating by contact with a system at higher temperature).
It is also possible to increase the internal energy of the system without increasing its temperature, for instance by a phase change.
In some processes a system may increase its temperature without receiving energy from the outside (such as in chemical reactions or in dissipative movements).
7 Thermal Phenomena, Radiation Energy, and Greenhouse Effect: A Teaching Learning Path
In this section we describe the activity sequence designed with the teachers. In the next one we discuss the implementation by the teachers (Sect. 8.1), we summarize the results obtained (Sect. 8.2), and present the new materials designed to improve the sequence (Sect. 8.3).
Greenhouse effect and climate change are complex topics involving many physical properties and phenomena (Onorato et al. 2011). To obtain an effective learning, it is necessary to proceed gradually, by focusing on each phenomenon involved, and to treat the greenhouse effect as a synthesis of the multifaceted aspects previously studied. To promote a stable and satisfactory understanding, it is not sufficient to simply explain at once what the greenhouse effect is. It is necessary to realize a series of activities, which can provide a well established background on which students can develop their own conceptual construction. To this aim, it is important to activate a synergic integration between qualitative experiments, theoretical systematizations, quantitative experiments and explicative models.
to distinguish between concepts and phenomena initially confused in a global undifferentiated notion (heat, visible radiation, infrared radiation);
to separate properties of the objects and characteristics depending on interactions (transparency, absorptivity, colour);
to develop the concept of stationary condition in non-equilibrium state, this is crucial for explaining many physical phenomena.
to distinguish the quantities temperature, energy, heat and work, and realize that it is possible to heat a body without giving heat and to give heat without heating the body;
to recognize and explain a stationary condition of temperature for objects exposed to sun or lamp radiation;
to differentiate heat and radiation and recognize that objects emit thermal radiation;
to understand that the behaviour of a material in its interaction with radiation depends on the considered region of spectrum (glass is transparent to visible but absorbs infrared thermal radiation);
to put together and coordinate the new knowledge acquired in order to understand the radiative greenhouse effect in a box-model;
to extend the model for understanding the greenhouse effect on Earth and the problem of climate change and global warming.
The complexity of the parameters involved in the explanation of the greenhouse effect requires the construction of simplified didactic models of the phenomena according to the particular student level. We have constructed a simplified model in analogy with solar greenhouses, which allows explain the whole phenomenon in a conceptually correct way even if it is approximated and incomplete (see below phase f).
Our teaching path includes lectures, laboratory and outdoor activities. Multiple experiences are suggested to favour a progressive construction of knowledge, from the simplest cases to the more complex ones, in a dialectic relation between hypothesis, experiments, refined observations and more complex explanatory frameworks. This approach can convey an idea of science as a human activity in continuous evolution and a dynamical conception of models and theories, thus helping pupils to distinguish between models and reality. Students are expected to understand that models offer only partial and limited explanations and that further and more precise analysis is required. This means that the construction of such a model fosters questions and allows new perspectives of inquiry.
To distinguish the quantities temperature, energy, heat and work, and realize that it is possible to heat a body without giving heat and to give heat without heating the body
To recognize and explain a stationary condition of temperature for objects exposed to sun or lamp radiation
Moreover the differences in the graphs for cylinders of different colour or transparency highlight the essential differences between thermal effects due to electromagnetic radiation and due to heat conduction. This leads to the problem of differentiating heat and radiation. In this phase the discussion should focus on the fact that radiation produces thermal effects when it is absorbed by the material, while it passes through without producing thermal effects when the material is transparent. Some students notice the peculiarity of this behaviour:
The cylinder will increase its temperature indefinitely
It will raise the maximum possible temperature according to the material
The object will increase its temperature until it will be full of energy, so that it cannot receive any more
and recognize the difference from what happens with heat: heat heats up the material when it passes through it, on the contrary, when radiation passes through the material it does not heat up.
The bottle does not heat up because it is transparent to the light and it lets the light go through; the glass cylinder allows the light to go through, therefore it reaches a lower temperature
To differentiate heat and radiation and recognize that objects emit thermal radiation;
To understand that the behaviour of a material in its interaction with radiation depends on the considered region of spectrum;
the emission spectrum of an object contains also invisible radiation beyond the red;
the emission spectrum of a source depends on the temperature (at temperature near to room temperature the emitted radiation is in the far infrared region);
optical properties of materials depend on the considered region of the spectrum (on the frequency or wavelength of the radiation);
some materials are transparent to visible light but absorb far infrared radiation.
It is now possible to resume the problem of the different maximum temperature reached by the two objects of different colour when exposed to the lamp radiation (higher for the black one than for the white, see phase b and Fig. 3).
We created a rainbow with the help of an overhead projector as a source of light and an optical prism. Using an infrared camera, we saw that there is “light” beyond the red colour of the visible spectre. We discussed about visible light, its wavelength range and about ultraviolet and infrared light. We established that we naturally cannot see infrared light, but we can observe it, using a camera. We “saw” infrared radiations emitted by a remote control through the camera, and using the same camera, we looked at a lamp with different current intensities and voltages.
To put together and coordinate the new knowledge acquired in order to understand the radiative greenhouse effect in a box-model;
RS solar radiation entering into the box;
RO radiation from the environment;
RLu ≅ RLd radiation emitted by the lid up and down;
RP radiation emitted by the plate;
QP and Q′P heat flux from the plate to the lid through the air inside the box and to the air outside through the walls;
QL heat flux from the lid to the air outside.
To extend the model for understanding the greenhouse effect on Earth and the problem of climate change and global warming.
The Earth surface is the analogue of the black plate: it absorbs the incident radiation and emits infrared radiation depending on its temperature.
The atmosphere plays a role similar to the lid: it is transparent to most solar radiation, but it absorbs most far infrared radiation emitted by the Earth, and emits infrared radiation depending on its temperature.
Absorption of the infrared radiation emitted by the Earth is due primarily to water vapour, clouds and CO2, with a smaller contribution from O3, N2O, CH4 and a little contribution of other anthropogenic gases, such as the chlorofluorocarbons like CFCl3.
In comparing the atmosphere to the lid of a greenhouse, it is necessary to remark that no heat flux can exit from the upper surface towards the space and that the thermal radiation coming from the outside space RO is absolutely negligible, so that QL, Q′P and RO must be disregarded. Moreover, the atmosphere temperature is not uniform along the vertical direction, so that RAd > RAu and this fact is represented qualitatively by a different width of the arrows. By writing the equations of the energy budgets of the Earth surface and of the atmosphere, students are led to the surprising result that radiation energy emitted by the atmosphere toward the Earth surface is a bit larger than the energy coming to the Earth surface from the Sun.
Result of the in and out energy fluxes is a mean stationary temperature on the Earth surface compatible with life. It is worth pointing out that the greenhouse effect has a fundamental role in keeping the temperature on the Earth’s surface warm enough for life. What it is potentially dangerous is an increase in this effect, the anomalous greenhouse effect, not the effect in itself. An increase of CO2 and of the other greenhouse gases in the atmosphere produces a variation of the energy budget of the Earth–atmosphere system with a consequent change of the mean global temperature so as to re-establish the equilibrium between the entering and exiting energy: to a decrease of R′E, corresponds an increase of R′′E, of RAd, and RAu. As a consequence, an increase of the temperature at the Earth surface takes place.
Based on the study of the simplified model of Fig. 12, the students can then analyze a representation like the one in Fig. 11, and verify the energy balances by using the numerical values given in this figure.
8 Using the Teaching Learning Sequence in High School Classes
8.1 Organization of the Work and Implementation by the Teachers
The sequence was implemented in 2010 in six high school classes, four with 17–18 years old students and two with 15–16 years old students, for a total of 121 students. We gave the teachers a document describing the teaching learning sequence and we discussed it in periodical meetings where the teaching plan of each teacher was progressively designed with the necessary variations due to the different class situations.
An initial and a final questionnaire. The questionnaires were designed by the researchers and the teachers in a collective work. Aim of the initial questionnaire was to obtain information on student’s ideas on thermal phenomena, thermal effects of radiation and greenhouse effect as result of previous instruction and of information from media and day life experience. The final questionnaire was on the same topics and contained some questions identical as the initial one, with others more related to the activity carried out by the students. They consisted of 6 and 7 open ended questions respectively.
Work sheets filled in by the students during the experimental activity. The worksheets included students’ predictions, experimental results, comparison between predictions and results, interpretations and conclusive remarks, relevant aspects of the experiments.
Video and audio taping of some class activities and videos produced by the students to document some of the experiments carried out.
Teachers’ reports on the class work collected in log books.
All documents have been discussed in periodical meetings with the teachers and uploaded in a web site devoted to the project.
Teachers prepared a final report where they described which elements of the proposed sequence they considered essential, how they passed from the research plan to their individual teaching plan and to the actual activity in classroom, and the results they observed at the end of the sequence. After having taught the sequence, the teachers continued to work with us in 2011 to restructure the teaching of thermal phenomena in their classes, to elaborate new materials for students and a teaching guide for the modified sequence (materials are available at http://fisica.unipv.it/pls/Materiali.htm - L’Energia e la sua conservazione).
Since the school situations were very different, teaching plans and methods presented relevant differences but were in line with the core of the sequence project, the cognitive progression, the aims, and the conceptual and methodological design.
We found it important to discuss with the teachers clearly and explicitly about the role of different elements of the sequence, by distinguishing a core of contents, conceptual correlations and methodological choices, which are essential, and a cloud of elements that can be re-designed or skipped by teachers. This core-clouds structure (Besson et al. 2010b) is useful both to permit teachers’ changes and to control them. In fact, physics education research has shown that teachers tend inevitably to modify research based proposals to fit them with the practical constraints of the class situation and with their personal preferences, attitudes and knowledge (Hirn and Viennot 2000; Pinto 2005; De Ambrosis and Levrini 2010). Our experience confirms how it is important to analyse and discuss these changes with the teachers and to compare them with the rationale and the general aims of the original proposal. This work usually produces an improvement and an enhancement of the original proposal thanks to the teachers’ contributions.
Students’ answers to the initial and final questionnaires and their comments in the worksheets were analyzed to gain information on the improvement of their understanding of energy concepts and of the greenhouse effect through the activity sequence. Answers were at first analyzed by each teacher and grouped in categories. Then the categories were discussed and revised by the whole group to obtain a shared categorization.
The answers show a progressively more correct and appropriate use of the concepts of heat, thermal conduction, radiation, temperature, internal energy. In particular, the idea of internal energy, almost non-existent at the beginning, becomes more and more a central element in students’ descriptions. Most of the students realized both the possibility of heating a body by using work or radiation and the difference between “heating” and “giving heat”. In the following we summarize the results obtained.
8.2.1 Realizing that Bodies at Any Temperature Emit Radiation
However, many students partly forget to consider thermal radiation in more complex systems such as the greenhouse box, where thermal radiation emitted by the plastic lid is often disregarded, even when the emission of the plate is correctly taken into account. This reveals students’ difficulty in controlling the coherence of their explanations when many aspects and parameters are involved in the physical situation. It appears the spontaneous tendency to simplify the descriptions by considering only one (or few) factor and forgetting the others, although already known and utilized in other simpler cases.
A hot iron emits a lot of radiation that vanishes when the iron is cooling down, and in any case it cannot be seen.
Ice emits very little, not visible radiation.
The black bottle heats up and cools down more than the others because black is a perfect absorber and also a perfect emitter.
Glass is transparent to visible radiation, but it is opaque to infrared rays.
8.2.2 Explaining a Stationary Condition of Temperature as a Consequence of Energy Balance
Nevertheless, we found a non negligible persistence of the idea of “saturation” (32 vs. 67 %): according to this idea materials have their own maximum possible temperature that once it is reached necessarily cannot increase yet. This fact can be interpreted as due to the slowness of the process needed to pass from explanations in terms of object properties to reasoning in terms of interactions and balances.
After a while, the temperature of the cylinders becomes constant because there is equilibrium between absorbed and given up radiation.
The black one reaches a higher temperature and in about 20 min the temperature is constant because there is equilibrium between absorbed and emitted radiation.
The temperature will be stable when the power emitted will equal the power absorbed.
We observed how the reasoning used by students was strongly influenced by the first presentation of the topic (examples, descriptions, approximations, explanations…); actually they tended to reuse, for the new situation, the same reasoning pattern used before, even if it was not appropriate. We call this tendency, observed also in other topics, the “imprinting” phenomenon in science learning (borrowing the term introduced by K. Lorenz in different contexts).
The cylinder absorbs heat from the sun, its temperature increases, then it reaches saturation and the temperature keeps constant.
Such explanations were not found in students of the classes where the analysis of thermal phenomena started just with the experiments dealing with the heating of bodies exposed to lamp or sun radiation. In this case, reasoning based on the idea of energy balance was more frequent to explain the difference between the constant temperature of the object and of the environment.
The bottles reach a thermal equilibrium between environment and system.
The condition of constant temperature is the condition of thermal equilibrium with the environment, heated by the lamp.
To avoid the “imprinting” effect it is useful to present since the onset a wide panorama of different situations and factors, in a qualitative and simplified way, in order to convey interpretations not limited to a particular aspect. For example, in introducing thermal phenomena it is useful to consider both situations of thermal equilibrium and stationary non equilibrium situations, in familiar simple cases. The idea that a stationary temperature condition does not entail thermal equilibrium is developed in the materials developed in 2011 where experiments, examples, and theoretical arguments are proposed to stress this aspect.
8.2.3 Understanding the Greenhouse Effect
Concerning the understanding of the greenhouse effect, the results confirm the importance of passing through all the cognitive steps of the teaching sequence giving enough time to each of them and spiraling back to the previous ones after a time, in a different context. Presenting the entire explanation of the greenhouse effect in a unique step is not effective; the phenomenon is complex and needs a progressive rapprochement.
However, many explanations still reveal some imprecision and confusion:
Because radiation emitted by the black plate (in the green house box) is blocked by the lid. This fact produces an increase of the temperature inside the green house. Moreover, the lid prevents convection from inside to outside and favours the increase of temperature inside.
Solar radiation passes through, but radiation emitted by the Earth does not pass.
Moreover, the idea of “trapping” of sun rays is still present (30 %).
Solar radiations pass through the transparent roof of a green house, but after they have hit the ground they change wavelength and cannot go out. It is the same mechanism of the Earth to keep a stable temperature; instead of the glass roof we have the atmosphere.
The rays, hitting the greenhouse, heat the material of the plate that gives up heat inside, but this heat cannot go out and then the temperature inside is higher than outside.
It is worthwhile to notice that some students recognised and explicitly remarked a change in their idea of “trapping”, but this change did not mean that they have completely abandoned such idea, thus confirming the persistency of this conception and the difficulty of a radical reasoning change:
Inside a greenhouse it is much warmer than outside because the transparent material lets part of the sun rays pass through, but they are trapped inside the greenhouse and warm it up.
Because it (the plastic cover) keeps inside a fraction of the radiation that produces an increase of the internal energy.
The increase of CO2 and greenhouse gases due to human activity in the atmosphere produces the greenhouse effect, which means trapping of part of sun radiation in the atmosphere which contributes to global warming. It is the same phenomenon that, in small-scale, happens in a greenhouse.
It is interesting to compare these data with the results we obtained with a group of 51 graduated student teachers: only 6 % gave a correct explanation, 35 % expressed the idea of trapping of sun rays and 31 % sustained that sun radiation enters and heats, but heat cannot get out or can get out partially by conduction and convection.
I was thinking that the radiation trapped in the greenhouse was the one emitted by the sun, on the contrary it is the one emitted by the plate.
8.3 Design of New Materials
Results confirm the basic choices inspiring the design of the cognitive path and its effectiveness for learning energy concepts. However, at the same time some critical elements in the teaching paths were pinpointed. The teachers recognized that the idea of energy budget and its “structuring” role to interpret the considered phenomena were not sufficiently stressed during the implementation of the sequence. Moreover, they acknowledged that the students’ conceptual difficulties in passing from the understanding of single parts of the sequence to a global understanding of the greenhouse effect were underestimated.
The students’ responses and teachers’ final reports showed the complexity of the topic and its manifold conceptual implications, and gave hints to respond to the research question: what type of materials, experiments, models and schematic representations can improve students’ understanding of this topic. Focusing on this question and on the obtained results, we revised the teaching sequence and prepared new materials for the students and a teacher guide. A source of difficulty underlined by the teachers was the lack of booklets for students designed to help them in deepening and systematizing the different topics also with a personal study, since textbooks appeared inadequate and sometimes misleading and the students’ notes were insufficient for the task.
We prepared three types of material: sheets for experiments; booklets for students’ personal study and deepening; suggestions for teachers.
Worksheets for experiments describe the objectives of the experiment, its role in the sequence, not the expected result, but the issues addressed. They include different phases: prediction, measurement, data collection and processing, interpretation of the results and final questions. Experiments are considered part of the scientific discourse on the topic, in a dialogue between theory, hypothesis and experiments. The booklets are not designed as handbook chapters or scientific popularizations, but to be adherent as much as possible to the experimental activities and to resume their particular aspects, by taking into account also what students have observed. Aims of these materials are to propose a discussion of the experiments, to clarify the results obtained, the problems stemmed, the conclusions that can be drawn by referring to the whole teaching path. Moreover, the material systematizes the results obtained in a theoretical framework coherent as much as possible though simplified and not definitive, also referring to other experimental results. Typical students’ difficulties and conceptions are taken into account and addressed with theoretical and experimental arguments. We tried to clearly differentiate between conclusions which derive directly from experiments and explanations involving microscopic models, based on entities such as atoms, molecules, electrons, quanta. The school level suggests teachers be cautious and to limit explanations to a descriptive phenomenological model. When structural models are introduced, we try to define the appropriate level of elaboration and detail and the theoretical reference frame, with drastic simplifications but without conceptual mistakes or general inconsistencies.
The revised path is divided in three phases in order to facilitate its insertion in the usual curriculum: T (thermal phenomena), R (radiation), and S (greenhouse effect). Moreover, we developed an introductory phase O to supply the knowledge on optics and waves necessary for the other phases and to introduce the wave model for light and electromagnetic radiation. This part can be used with students who did not study the topic before or proposed to recall topics already studied.
Phase T is a crucial one, different from the usual approach to heating and cooling phenomena proposed in textbooks. We suggest to consider with students, since the beginning, different way to increase the body temperature (mechanical work, friction, heat, chemical reactions, lamp and microwaves radiation), and to show that it is possible to heat bodies without giving heat and to give heat without heating. Experiments are proposed and discussed where thermal equilibrium is reached and others where stationary temperature conditions are attained without thermal equilibrium, due to the energy fluxes balance, also referring to everyday life examples. A precise theoretical differentiation between thermal equilibrium conditions and stationary non equilibrium conditions is given. Descriptions based on the energy concept are used and energy is introduced as a quantity useful to quantitatively connect different phenomena and to describe processes and transformations involving different empirical areas. The link between internal energy and temperature is introduced in a qualitative or semi-quantitative way.
Phase R deals with the dependence of the properties of radiation-matter interaction (absorption, reflection, transparency, and emissivity) on the radiation frequency. The aim is to pass from the idea of radiation as a whole to the idea of spectrum with a distinction in different types, and in particular between visible and infrared radiation. The experiments presented in Sect. 7 are described and discussed and the temperature versus time graphs of bodies exposed to sun or lamp radiation are accurately analysed by distinguishing the almost linear initial part, its bending, and the stationary final part, which is a key point for understanding the following phase S.
Phase S aims at integrating the knowledge acquired in the previous work in order to give a coherent and correct interpretation of the greenhouse effect in a box-model and of the greenhouse effect on the Earth so that the general problem of global warming can be discussed.
9 Concluding Remarks
Energy is a central concept in physics and is a key concept for understanding physical, biological and technological world. Many researches in science education have studied how to present energy concepts and phenomena, different approaches have been proposed and tested, while students’ difficulties and conceptions have been investigated. Energy is a complex topic with multiple connections with different science areas and with social, environmental and philosophical issues.
The choice of a particular teaching approach is not a direct consequence of technical content knowledge, but involves implicit or explicit epistemological assumptions on learning processes, on the role of science teaching in cultural development, on the nature of science and on its connections with social and environmental issues.
However, in order to produce effective teaching/learning sequences, it is necessary to design teaching paths based on research work and empirical observations. As our experience also showed, even activity sequences and teaching methods that appear properly organized and structured to the designer may reveal ineffective and problematic for students and unable to address their difficulties. For that reasons well structured experimentations with students are necessary.
In this paper we have discussed some aspects of the teaching and learning of energy concepts, and we have reported results of research in science education on this issue. Then we have described a teaching learning path, devoted to high school students, that has been developed around a specific driving issue, greenhouse effect and global warming, to promote the progressive construction of physics concepts and models.
Using the sequence with high school students has shown that the approach based on driving issues promotes students’ engagement toward a deeper understanding of the topic and favours further insight.
Data collected allowed us: (1) to focus on the limitations in the traditional way of dealing with thermal phenomena and optics as separate topics, not connected with the general energy problem, (2) to identify with more detail particular students’ difficulties on thermal effects of radiation-matter interaction and on the interpretation of stationary conditions in terms of energy balances.
Analysis and discussion with the teachers of the class work results was followed by a revision of the teaching sequence and by the production of new materials for students and for teachers. These materials are the result of a didactical reconstruction of the content matter realized by the group of teachers in collaboration with our research group starting from the results obtained with the students. Thus, a new cycle of implementation, evaluation and refinement has been activated this year with a larger group of teachers and students in order to take into account effects and elements of the materials that only a contextualised practice can reveal. We think that this type of systematic and long term collaboration with teachers can help to fill the gap, widely recognized, between the results of science education research and the actual school practice.
As an example: “In this process… two electrons become pure energy because they annihilate with positrons (anti-electrons)… As a spirit animating what exists around us, energy is in everything, sometimes tangible as the Sun light, sometimes hidden in remote corner of reality… All bodies producing a field are subjected to its action, thus all bodies, being no more than energy lumps, interact gravitationally the one with the other” (translated by the authors from De Felice F. (2000), pp. 13, 23, 24).
See Boyes and Stanisstreet (1993), Rye et al. (1997), Groves and Pugh (1999), Koulaidis and Christidou (1999), Meadows and Wiesenmayer (1999), Andersson and Wallin (2000), Papadimitriou (2004), Osterlind (2005), Lester et al. (2006), Kilinc et al. (2008), Svihla and Linn (2012).
We used the GLX data loggers by PASCO. We also used the metal cylinders, the metal bottles and the plastic box provided by PASCO for the experiments described in the following.
We would like to thank all the students and teachers who participated in this work. Research was partly supported by the National Project Piano Lauree Scientifiche (Degrees in Science Project) funded by the Italian Ministry of Education.